A soft or semi-permanent virtual circuit (“SPVC”) is a generic term for any communications medium that is permanently provisioned at the end points, but switched in the middle. An example of an SPVC is illustrated in FIG. 1. An SPVC through an asynchronous transfer mode (“ATM”) network comprises permanent virtual circuits (“PVC”) 100 at the entry 110 and exit 120. A switched virtual circuit (“SVC”) 130 connects the entry 110 and exit 120. Individual switches 140 or nodes allow for different switched virtual paths (“SVP”) to connect the entry 110 and the exit 120. At the entry (originator) 110 device and the exit (target) 120 device, the PVC is cross-connected to the SVC.
In a private network-network interface (“PNNI”) network provisioned with primarily SPVC connections, the demand on control resources remains low until significant changes are precipitated by a failure in the network. A link or node failure may necessitate the rerouting of thousands of SPVCs. This activity results in a burst of call release, setup, route selection, and call connect activity as connections are released (de-routed) and rerouted over alternate paths, all in a relatively short period of time.
When a trunk fails at the PNNI, call clearing is initiated on both sides of the failed trunk by sequentially sending release messages for every failed connection. Once the SPVC manager 150 at the master endpoint 120 receives a release, the SPVC manager initiates a reroute of the connection by sending a setup message into the network. In an effort to conserve switch resources, the SPVC manager reacts to local node congestion, avoiding the sending of setup messages to an already congested switch.
Simulations of up to 100 node networks with failures of links carrying tens of thousands of connections provide evidence that large numbers of setup messages can be dropped at the switches at or near each end of the failed link. These drops, and the ensuing retries, significantly increase the time required for the network to recover from a failure. In these scenarios, reacting to local node congestion yields only a moderate effect on helping the SPVC manager send reroute messages into the network without causing congestion at a distant node. Although local switch congestion may be abated at the connection end nodes, evidence suggests that congestion will not be avoided in the core of the network.